Refactor pound spin

[Koopa1018]

Description of changes

Moves the pound spin animation from player.gd to character_sprite.gd. The effect is purely visual (can't affect FLUDD angle since you can't FLUDD during a pound), so it probably shouldn't be lumped in with the player physics.

Not to be confused with the pound spin rise animation, which does affect player physics. See #247 for that.

Issue(s)

Makes good on a longstanding TODO in the code.

Before merge:

• [ ] There's an annoying pop forward on the X axis when the spin ends (also brought up by Kuma in #241). I want to fix that, but I'm still not 100% sure how to go about it. (The offending line has at least been identified.)

[GTcreyon]

I suspect this will be obsoleted by #244. Should this be closed?

[Koopa1018]

Hmm, yeah, good call